Horrible, horrible, stupid idea. I have zero interest in mirrorless cameras, period. Having seen the tiny little Sonys in the flesh, they appear to be made for small Asian women's hands, actually. The Canon EOS R is larger, but still much smaller than my 5DsR. The 5DsR fits my hands, and offers a good mounting point for big lenses such as the 11-24L and 100-400L I have, among other L lenses. Using these lenses on a much smaller body would be far less ergonomic. (I also hand hold the 5DsR quite often in bright light with very good results, contrary to what many "experts" said about it after its release.)
I also want the optical viewfinder. Other than making the camera smaller and lighter, which I don't want or need since I don't have dainty hands and/or weak arms, mirrorless offers me nothing. I don't see ever running out of shutter actuations based on my upgrade times either.
I wonder what % of pros, men in particular, really are super interested in more dainty cameras just for the novelty and size/weight difference of mirrorless. If Canon opts to take its current megapixel king down to what I would call a gimmick/trend camera, I'll be concerned that DSLRs may be headed the route of the high end stereo equipment of the 1970s and 1980s. For the most part high end stereo options are few are far between compared to what they used to be. There is enough pressure on DSLR sales already via smartphones and other cheaper options. Trying to mitigate that by forcing current model lines to mirrorless options is kind of like Windows 8 and 8.1 being launched as a response to Android and iOS, where Microsoft thought desktop users would be fine with a smartphone style OS. Another horrible idea, and those 2 options bombed horribly in fitting fashion. (Win 10 has only overtaken Win 7 recently due to free upgrades to Win 10 and the looming axe of no future updates forcing the corporate world to switch over.)
If Canon has decided it needs to offer more mirrorless to respond to Sony and NIkon, without compromising the feature set of exiting DSLRs and certainly without changing any 5D models to mirrorless, fine. Go for it, but get the 5DsR Mark II released within the next 3 to 6 months with useful upgrades and no AA filter at all, and with the mirror and existing form factor intact.